1. Field of the Invention
The present invention relates to a cone beam artifact reduction method for a half reconstruction method or the like which is higher (shorter) in time resolution than full reconstruction in cone beam helical reconstruction and, more particularly, to an X-ray computed tomography apparatus which obtains in-vivo information as an image by applying X-rays to a living body.
2. Description of the Related Art
In cone beam CT, as is known, when circular orbit acquisition is executed, radon data necessary to obtain a complete solution cannot be acquired, and more cone beam artifacts appear with an increase in the distance from the central plane. Consider half reconstruction in cone beam circular orbit scanning. Half reconstruction is characterized by being shorter (better) in time resolution than full reconstruction. However, half reconstruction is implemented with a fan beam but is not implemented with a cone beam. As shown in FIG. 10, with a fan beam, since so-called counter data exist, it is possible to obtain a complete solution if there is (180°+fan angle) data (half reconstruction).
On the other hand, as shown in FIG. 11, in cone beam CT, counter data do not pass through the same path except on the central plane due to the influence of the cone angle. This leads to the omission of radon data. Consequently, more noticeable cone beam artifacts appear than in full reconstruction, resulting in a deterioration in image quality.
As described above, in half reconstruction in cone beam reconstruction, the problem of cone beam artifacts is more noticeable. As a conventional technique, there has been proposed a technique of balancing a time resolution and artifacts by, for example, assigning a weight to one-rotation data instead of half-rotation data. However, such a technique is not a fundamental measure against “the omission of radon data”, and is a technique which can at best change the manner of how artifacts are seen. That is, a fundamental solution has not been achieved.
Recently, the Line+Circle method has been proposed by Katsevich, the University of Central Florida. This technique is designed to obtain a complete solution by adding line data to half scanning. A prerequisite of this technique is that line data is in contact with the scan start or end point of half scanning. In the case of dynamic half reconstruction in dynamic scanning, this prerequisite does not hold. Obviously, this problem can be solved by acquiring many line data at each tube position. However, this technique is not practical in consideration of actual operation. In the case of segment reconstruction in which half-scanning data is obtained by using a plurality of heartbeat data obtained in ECG-gated reconstruction, a data group constituting half-scanning data sometimes exists discontinuously in the tube position direction. In such a case, the proposal by Katsevich cannot be used.
As described above, with regard to cone beam artifacts, the half reconstruction method is superior in time resolution at the sacrifice of image quality.